The present invention relates to a current-limiting circuit and a constant voltage source for the current-limiting circuit, and particularly to a current-limiting circuit that is used to control the output current from a power semiconductor element, which element is in turn controlled by input signals to a control terminal made of, for example, a metal oxide semiconductor field effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT) or a bipolar power transistor.
A current-limiting circuit designed to prevent breakdown of power semiconductor elements due to an excessive current, or an "overcurrent," and to control current flowing into a load at a constant value, is known in the art. A protective circuit for preventing a breakdown caused by an overcurrent, which circuit is shown in FIG. 2, is described by D. L. Zaremba Jr. in "How Current Sense Technology Improves Power MOSFET Switching Applications," Electro. Mini/Micro Northeast Conf. Rec., E. 10/4, pp. 1-4 (1986). In this circuit, an N-channel MOSFET 12 acts as a current mirror element through which flows a shunt current (sense current) I.sub.S which is proportional to an output current I.sub.D flowing through an N-channel power MOSFET 11 acting as a power semiconductor element or a main semiconductor element. MOSFET 11 has its drain terminal and gate terminal in common with a drain terminal 21 and gate terminal 23 of the MOSFET 12. A current-detection resistor 31 is connected between a current mirror terminal 24, which acts as the source terminal of MOSFET 12, and a source terminal 22 of the MOSFET 11. A constant-voltage source 5 has its positive output terminal 51 connected to a negative terminal 41 of an operational amplifier 4, and a negative output terminal 52 connected to source terminal 22 of the MOSFET 11. The current mirror terminal 24 is connected to a positive input terminal 42 of the op-amp 4, and an overcurrent-signal input terminal 62 of a drive circuit 6 is connected to an output terminal 43 of the op-amp 4. Furthermore, the common gate terminal 23 of the MOSFETs 11 and 12 is connected to a drive output-voltage-output terminal 61 of the drive circuit 6 via a gate resistor 32.
The current-limiting circuit shown in FIG. 2 operates in the following manner. When the output current I.sub.D increases, the sense current I.sub.S also increases proportionately. As a result, the voltage across the current-detection resistor 31 also increases. When this voltage exceeds the output voltage from the constant-voltage source 5, the output terminal 43 of the operational amplifier 4 goes into a high state and causes the drive circuit 6 to sense an overcurrent condition at the connected overcurrent-signal input terminal 62, and the drive circuit turns the output stage transistor 11 off by putting the drive-voltage-output terminal 61 into a low state. These operations prevent overcurrent breakdown of the power semiconductor element 11.
Although the circuit shown in FIG. 2 can prevent an overcurrent breakdown of a power semiconductor element, it cannot maintain a constant current level in the load. The current-limiting circuit shown in FIG. 3, which achieves the same objective as the circuit of FIG. 2 and is also described in the above-mentioned article by Zaremba Jr., addresses this constant-current control problem. Parts shown in FIG. 3 which are identical to those shown in FIG. 2 are given the same numeral. In this circuit, the current mirror terminal 24 is connected to the negative input terminal 41 of the operational amplifier 4, the positive output terminal 51 of the constant-voltage source 5 is connected to the positive input terminal 42, and the common gate terminal 23 is connected to the output terminal 43 of the op-amp 4 via a gate resistance 33.
This current-limiting circuit operates in the following manner. When the output current I.sub.D is small, the sense current I.sub.S is also proportionally small, and the voltage between both ends of the current-detection resistor 31 is low. In contrast, when the output current I.sub.D is large, the voltage between both ends of the current-detection resistor 31 is high. Therefore, when the output current I.sub.D is small and the voltage Vs across the current-detection resistor 31 is lower than the output voltage V.sub.REF of the constant-voltage source 5, the output terminal 43 of the operational amplifier 4 produces a sufficient voltage to maintain sufficiently high electric conductivity in the normally-turned-on, output-stage transistor 11.
Next, when the output current I.sub.D increases and the Vs becomes greater than the V.sub.REF, the output voltage at the output terminal 43 of the operational amplifier decreases and the electric conductivity of the output-stage transistor 11 decreases because of a drop in the voltage applied to the gate terminal 23. As a result, it is more difficult for the output current I.sub.D to flow, and the current is maintained below a certain value.
The current-limiting circuit shown in FIG. 3 has two drawbacks. One is an oscillation phenomenon. The output from the operational amplifier 4 is fed back to the negative input terminal of the operational amplifier 4 through the current-mirror transistor 12. The gain in this feedback loop is nearly equivalent to the gain in the operational amplifier 4, which is normally close to 100 dB. In addition, a phase delay occurs at the output terminal 43 of the operational amplifier 4, which carries as large a load as the output-stage transistor 11. As a result, a feedback loop that can easily cause a high gain with a phase delay is formed in the current-limiting circuit 3, allowing the oscillation phenomenon to occur more frequently.
Another drawback of the circuit shown in FIG. 3 is that, because the current is maintained at a constant value, the circuit cannot be used to maintain multiple levels of constant current.
The constant-voltage source 5 shown in FIGS. 2 and 3 has been described in detail in "Bipolar and MOS Analog Integrated Circuit Design" written by A. B. Grebenn and published by John Wiley Sons, New York.
Other current-limiting circuits such as band-gap reference circuits and circuits using constant voltage characteristics in Zener diodes are also known. However, the band-gap reference circuit has disadvantages of increased device size and costs. The circuit utilizing constant voltage characteristics in Zener diodes has a disadvantage of the voltage deviating from the constant voltage if it drops below the breakdown voltage, which is normally 6 V to 8 V in Zener diodes.
It is an objective of the present invention to solve the above problems and provide a current-limiting circuit that exhibits no oscillation phenomenon.
It is another object of the present invention to provide a current-limiting circuit that can be used to control constant currents at multiple levels.
It is yet another object of the present invention to provide a low cost, constant-voltage source for a current-limiting circuit which can be used with low supply voltage.